The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with gradient coils for a magnetic resonance imaging apparatus and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with localized magnetic resonance spectroscopy systems and other applications which utilize gradient magnetic fields.
In magnetic resonance imaging, a uniform magnetic field is created through an examination region in which a subject to be examined is disposed. A series of radio frequency pulses and magnetic field gradients are applied to the examination region. Gradient fields are conventionally applied as a series of gradient pulses with pre-selected profiles. The radio frequency pulses excite magnetic resonance and the gradient field pulses phase and frequency encode the induced resonance. In this manner, phase and frequency encoded magnetic resonance signals are generated.
More specifically, the gradient magnetic field pulses are typically applied to select and encode the magnetic resonance with spatial position. In some embodiments, the magnetic field gradients are applied to select a slice or slab to be imaged. Ideally, the phase or frequency encoding uniquely identifies spatial location.
In bore type magnets, linear magnetic field gradients are commonly produced by cylindrical gradient field coils wound on and around a cylindrical former. Discrete coils are wound in a bunched or distributed fashion on a similar or larger diameter cylindrical tube, commonly 30-65 centimeters in diameter or larger.
Historically, gradient coil designs were developed in a "forward approach," whereby a set of initial coil positions were defined and the fields, energy, and inductance calculated. If these quantities were not within the particular design criteria, the coil positions were shifted (statistically or otherwise) and the results re-evaluated. This iterative procedure continued until a suitable design was obtained.
Recently, gradient coils are designed using the "inverse approach," whereby gradient fields are forced to match predetermined values at specified spatial locations inside the imaging volume. Then, a continuous current density is generated which is capable of producing such fields. This approach is adequate for designing non-shielded or actively shielded gradient coil sets.
Often, shielded gradient coil sets are designed such that their gradient magnetic field has an inherent rollover point along, but near the outer edge of its perspective gradient axis. That is, the first derivative of the gradient magnetic field is zero at a certain location along the gradient axis and inside the physical volume bounded by the inner surface of the gradient structure. The gradient magnetic field takes on non-unique values after passing the rollover point where the first derivative of the gradient magnetic field is zero. The rollover point may be in the center or near the edge of the bore, beyond where the subject is positioned. This design is problematic for an imaging sequence with a large field of view (FOV) in which portions of the subject are disposed between the rollover point and the bore. Areas of a subject that are located beyond the rollover point will alias back into the image, which causes ghosting and distortion of the image. A gradient deghosting and distortion algorithm is then implemented during postprocessing in order to compensate for distortions in the image. The gradient distortion algorithm, particularly when applied to all three gradient coils, extends the image postprocessing time and extends significantly the overall time of the magnetic resonance study.
In addition, information in the raw data related to the voxels located beyond the rollover point cannot be recovered uniquely. Voxels on either side of the rollover point that experience the same gradient strength are encoded indistinguishably. This limits the maximum FOV of a given sequence and limits the range of translational movement for the examined subject inside the image volume. This problem is particularly apparent when imaging extremities, such as shoulder, wrists, legs, and elbows, because typically these regions are located near the rollover point. Therefore, any attempt to move one side of an extremity near the isocenter of the imaging volume places the other side in the vicinity of the rollover point, which results in the aforementioned aliasing problems.
The present invention contemplates a new and improved gradient coil set which overcomes the above-referenced problems and others.